Physiological cooling agents are constantly gaining ground in various consumer applications due to their ability of improving desirable sensate properties of the product. These sensate properties are generally explained by the chemical action of these compounds on nerve endings responsible for sensation of cold. Common applications and uses of these compounds include foods, beverages, dentifrices, mouthwashes, toiletries, lotions, confectionery, and pharmaceutical products.
Being a major constituent of peppermint oil, menthol has been used in several of the above mentioned applications for a very long time. Menthol is a physiological cooling agent and is well known for its analgesic, freshening and flavouring effects on the skin and/or the mucous membranes of the mouth. A strong initial cooling sensation can be achieved if the products containing menthol are applied to the skin and/or mucous membrane. However, very often there is a need for products containing a coolant composition in order to provide a long lasting cooling sensation.
Menthyl lactate is also an important ingredient that is valued for its cooling properties. Compared to menthol, menthyl lactate has the advantages of low irritation, long lasting cooling effect and low volatility. Nevertheless menthyl lactate does not produce an initial cooling effect as strong as menthol and therefore its use has been limited to some extent, especially for oral care and skin care products, where the user desires to experience a strong cooling effect as soon as the product is applied. A need therefore was felt for a composition which will contribute the long lasting cooling sensation to a product without the unwanted harshness and unpleasant odor that come from menthol. Various cooling agents have been tried independently or in combination with another agent. The search for more and more cooling and flavouring agents is going on; see WO 00/142983, JP 0632528, JP 10231238, WO 2004/1037764.
Currently, compositions of menthol and menthyl lactate used in the market are prepared by physical mixing of the two components in the required ratio (hereafter called “Artificial Mixture”). The disadvantage thereof is that the menthol and menthyl lactate have to be either melted first at a temperature of 50-60° C. or dissolved in perfume oils, cosmetic oils or glycol solvents such as 1,2-propylene glycol. Furthermore, menthol being slightly volatile, evaporates and generates a strong minty odor which causes unfavorable health and safety issues besides the loss of menthol. Menthyl lactate is available from Symrise GmbH (Germany) in two forms: a crystalline and a non-crystalline form. Each of these solids has the disadvantage of being used in solid form. In case of a non crystalline solid, its container has to be heated in order to transfer the solid from the container whereas in a crystalline solid the presence of 0.1% sodium bicarbonate may cause compatibility problems in the formulation and may need to be removed.
This shows that menthol and menthyl lactate, if added separately in their respective normal solid form to the formulation, are not convenient. This is not favorable for large scale industrial manufacturing processes. On the one hand, such heating process needs to have specific equipment and heating facilities to melt the menthol and menthyl lactate. On the other hand, menthol and menthyl lactate, if added separately in their solid form, are not suitable for cold processing technologies which are a growing trend in the manufacturing in order to save energy and improve productivity. Furthermore, when heated up, menthol being low melting evaporates and generates a strong minty odor that is disagreeable and causes unfavorable health issues, besides the loss of menthol.
The problem to be solved by the invention is the provision of a composition of menthol and menthyl lactate which is liquid at room temperature and whose crystallization point is much below room temperature. Such a composition could be added directly in liquid form into a desired formulation which makes the formulation process more convenient without raising any health and safety issues.
There are reports in the technical literature where menthol and menthyl lactate have been used in the same formulation by adding menthol and menthyl lactate separately and there is no mention about eutectic mixture formation in this formulation.
US 2004/10018954 describes compositions containing menthol and menthyl lactate in different proportions in the range of 1:4 to 4:1 and its effect on the freezing point of the mixture. It states that menthol and menthyl lactate can be physically mixed in different ratios and stirred for 20 minutes to get a clear liquid having a crystallization point below room temperature. But it was observed that it is very difficult to stir two solids and it also takes a long time (sometimes even 24 hours) to obtain a clear liquid.
WO2007/044146 describes a method for producing menthyl lactate from menthol and lactic acid. The method disclosed therein aims at as high as possible yields of ML. Maximizing the overall yield of ML is emphasized in the summary section of said description. The skilled reader is expressly instructed to take measures to avoid that too much of the ML is being converted to menthol. All working examples disclose contents of unreacted menthol of less than 8%. This illustrates that almost complete conversion of menthol to ML and obtaining pure ML are key features of the teaching of WO2007/044146.